What I expected to happen was...:
After removing the enchantment from a book with the grindstone the disenchanted book would stack with regular books.

What actually happened was...:
Even though regular books and disenchanted books both look the same and are called "book", they do not stack together. Note that disenchanted books do stack together, they just don't stack with books that have not been enchanted before.

Steps to Reproduce:
1. Get an enchanted book
2. Remove the enchantment from it using a grindstone
3. Attempt to stack the disenchanted book with some "regular" books.